Magnetic recorder and magnetooptical recorder

ABSTRACT

Virtual tracks are set so that the rotation center of a magnetic recording medium or the vicinity of the rotation center coincides with the track center of the data information, and positioning servo control is performed. Thus, since it is not necessary to faithfully follow the track center line specified by the servo information that is previously recorded on the magnetic recording medium or magnetooptical recording medium, a vibration is not easily caused in synchronism with the rotation. Therefore, when the rotation center and the pattern center are deviated from each other, since a vibration is not caused in synchronism with the rotation, the magnetic head of the magnetic recorder or the magnetooptical head of the magnetooptical recorder is able to follow tracks with higher precision. Accordingly, the recording density can be increased.

TECHNICAL FIELD

The present invention generally relates to magnetic recorders andmagnetooptical recorders, and particularly to a magnetic recordingmedium or magnetooptical recording medium having servo informationwritten on its data area, and to a magnetic recorder or magnetoopticalrecorder in which high tracking precision can be achieved in cooperationwith the medium.

BACKGROUND ART

The so-called "embedded servo" type magnetic disk apparatus employs sucha disk as to have servo information written on the data area in orderfor the information to cause the magnetic head to follow a desired datatrack. The magnetic disk used in the apparatus of this type has aplurality of servo sectors for servo information and a plurality of datasectors for data alternately arranged on the data area (for example, seeJP-B-55-20302).

In the conventional magnetic disk of this type, after servo informationis recorded on the magnetic disk, the magnetic head is located with highfidelity at the center of a track based on the servo information, and itrecords and reproduces the data information on and from the track.

Also, in the magnetooptical disk apparatus, after the servo informationis transferred to the magnetooptical disk by photolithography or thelike, the magnetooptical head is located with high fidelity at thecenter of a track based on the servo information, and it records andreproduces the data information.

In the above conventional head-positioning method, however, if thecenters of tracks at which the magnetic head or magnetooptical head islocated on the basis of the servo information have initialeccentricities with respect to the rotation center of the disk, sincethe magnetic head or magnetooptical head tries to follow the trackcenter lines with high fidelity and to record or reproduce datainformation, vibration will occur which synchronizes with the diskrotation while the head is being moved to follow the tracks. Thisvibration problem will be described with reference to FIG. 1(a).

FIG. 1(a) schematically shows the deviation e between a rotation center11 of spindle for a magnetic disk 10 and a center of circles (patterncenter) 12 which are formed by the center lines of tracks that arespecified by the servo information recorded on the magnetic disk 10. Asillustrated in FIG. 1(a), the rotation center 11 of spindle on themagnetic disk 10 is assumed to be deviated by e from the center 12 ofcircles of the servo pattern. Accordingly, the center line 13 of m-thdata track and center line 14 of (m+1)-th data track specified by theservo pattern are decentered while the disk is being rotated. Forexample, the track center 13 is detected according to the servo patternpresent within each of the n-th servo sector 15a, (n+1)-th servo sector15b, (n+2)-th servo sector 15c and (n+3)-th servo sector 15d (thefollowings are omitted) of the m-th data track, so that a locus whichthe head 16 is required to trace can be found. In this case, when themovement of the head 16 is observed from the outside of the disk (, orwhen it is plotted on absolute coordinates), a sinusoidal undulation canbe seen as shown in FIG. 1(b) at 19a. The amplitude of this undulationis represented by e. When the movement of the head 16 is observed fromthe disk 10 (, or when it is plotted on relative coordinates), however,a sinusoidal undulation can also be seen of which the amplitude is 0 ifthe head can perfectly follow the track center 13 or e/100 if thesuppression ratio of the head servo system around the revolution rate isset for about -40 dB in the apparatus, as shown in FIG. 1(c). Therefore,since the head is positioned to be substantially zero in its relativeposition as shown in FIG. 1(c) at 19c, it vibrates on the absolutecoordinates as illustrated in FIG. 1(b) at 19a.

The head system itself moves according to the above vibration, thusexcites the mechanical system to vibrate, and finally decreases thepositioning precision itself.

Thus, the conventional magnetic apparatus has the problem that when therotation center of the disk is deviated from the pattern center, avibration is caused in synchronism with the disk rotation.

Accordingly, it is an object of the invention to provide a magneticrecorder capable of preventing a vibration from being caused insynchronism with the rotation of the magnetic disk when the rotationcenter of the magnetic disk is deviated from the pattern center.

For the same reasons, the conventional magnetooptical disk recorder alsohas the problem that when the rotation center of the disk is deviatedfrom the pattern center, a vibration is caused in synchronism with thedisk rotation.

Accordingly, it is another object of the invention to provide amagnetooptical recorder capable of preventing a vibration from beingcaused in synchronism with the rotation of the magnetooptical disk whenthe rotation center of the magnetooptical disk is deviated from thepattern center.

DISCLOSURE OF INVENTION

(1) A magnetic recorder of the invention includes a magnetic recordingmedium having servo information and data information magneticallyrecorded, a magnetic head for recording or reproducing thoseinformation, means for driving the magnetic recording medium to rotatearound a certain point on the medium, means for moving the magnetic headon the surface of the magnetic recording medium on the basis of theservo information, means for determining an eccentricity between therotation center of the magnetic recording medium and the center ofcircular servo tracks on the basis of the servo information, means forgenerating deviation data of tracks from the determined eccentricity,means for storing the deviation data, and means for setting virtualtracks so that the rotation center of the magnetic recording medium orthe vicinity of the rotation center coincides with the track center ofthe data information on the basis of the stored information, and makingpositioning servo control on the basis of the virtual tracks.

The eccentricity is calculated on the basis of, preferably, the servoinformation equally and discretely arranged in the rotation direction ofthe magnetic recording medium.

The eccentricity is also calculated on the basis of, preferably, aspecial pattern arranged on part of the magnetic recording medium.

The deviation data is formed of, preferably, sector informationindicating sector positions, positional error information indicatingtrack center position, and deviation information at the sectorpositions.

In addition, another magnetic recorder of the invention includes meansfor generating deviation data of tracks on the basis of the servoinformation, means for storing the deviation data, and means for settingvirtual tracks so that the rotation center of the magnetic recordingmedium or the vicinity of the rotation center coincides with the trackcenter of the data information, and making positional servo control onthe basis of the virtual tracks.

The deviation data is formed of, preferably, sector informationindicating sector positions, positional error information indicatingtrack center position, and track information indicating track positions.

The means for storing the deviation data is preferably a volatilesemiconductor memory provided within the magnetic recorder.

The means for storing the deviation data is also preferably anonvolatile semiconductor memory provided within the magnetic recorder.

The deviation data is preferably magnetically stored on the magneticrecording medium.

The center line of each of the virtual tracks coincides with,preferably, a locus of the magnetic head on the magnetic recordingmedium with a voice coil motor that is used for moving the magnetic headbeing fixed.

The virtual tracks are set so that the rotation center of the magneticrecording medium or the vicinity of the rotation center coincides withthe track center of the data information on the basis of the storeddeviation data, followed by positioning servo control, and preferably,when the rotation center of the magnetic recording medium is shiftedafter the virtual tracks are set, new virtual tracks are set, followedby positioning servo control.

When the rotation center is shifted, preferably the data information istransferred to a continuous free region on the magnetic recordingmedium, and then new virtual tracks are set.

In addition, when the rotation center is shifted, preferably the datainformation is transferred to the semiconductor memory provided withinthe magnetic recorder, new virtual tracks are set, and then the datainformation is written back.

Also, when the rotation center is shifted, preferably the datainformation is transferred to the continuous free region on the magneticrecording medium or to the semiconductor memory provided within themagnetic recorder, the eccentricity is measured, and then the datainformation is read in while feed-forward control is being made on thebasis of the calculated deviation data.

Moreover, there is provided another magnetic recorder including amagnetic recording medium having servo information and data informationmagnetically recorded, a magnetic head for recording or reproducingthose information, means for driving the magnetic recording medium torotate around a certain point on the medium, means for moving themagnetic head on the surface of the magnetic recording medium accordingto the servo information, means for determining an eccentricity betweenthe rotation center of the magnetic recording medium and the center ofcircular servo tracks on the basis of the servo information, means forgenerating deviation data of tracks from the determined eccentricity,means for storing the deviation data, and means for making positioningservo control relative to an initial position of the magnetic head withthe head fixed at the initial position without offset addition(correction), which initial position is treated as the storedinformation.

The initial position is preferably set in association with a particularposition on the surface of the magnetic recording medium.

The initial position is also preferably set in association with allnecessary regions on the surface of the magnetic recording medium.

(2) A magnetooptical recorder of the invention includes a magnetoopticalrecording medium having servo information and data information recordedin magnetooptical way, a magnetooptical head for recording orreproducing those information, means for driving the magnetoopticalrecording medium to rotate around a certain point on the medium, meansfor moving the magnetooptical head on the surface of the magnetoopticalrecording medium on the basis of the servo information, means fordetermining on the basis of the servo information an eccentricity thatoccurs in association with the rotation of the magnetooptical recordingmedium, means for generating deviation data of tracks from thedetermined eccentricity, means for storing the generated deviation data,and means for setting virtual tracks so that the rotation center of themagnetooptical recording medium or the vicinity of the rotation centercoincides with the track center of the data information on the basis ofthe stored deviation data, and making positioning servo control.

In the magnetooptical recorder, the virtual tracks are set so that therotation center of the magnetooptical recording medium or the vicinityof the rotation center coincides with the track center of the datainformation on the basis of the stored deviation data, followed by thepositioning servo control, and when the rotation center of themagnetooptical recording medium is shifted after the virtual tracks areset, new virtual tracks are set, and the positioning servo control isperformed.

In the magnetic recording medium of the invention, since the head neednot faithfully follow the track center line specified by the servoinformation recorded on the magnetic recording medium or magnetoopticalrecording medium, a vibration synchronized with the rotation is noteasily caused.

Therefore, when the rotation center and the pattern center are deviatedfrom each other, since a vibration synchronized with the rotation of thedisk is not caused, the magnetic head or magnetooptical head can bepositioned with extremely high precision.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(a) is a plan view of main part showing the idea of virtual tracksin embodiment 1 of a magnetic recording medium according to theinvention; FIG. 1(b) is a graph showing the locus of the head when themovement of the head in the radius direction of the disk is observedfrom the outside of the recorder; and FIG. 1(c) is a graph showing thelocus of the head when the movement of the head in the radius directionof the disk is observed on the disk surface.

FIG. 2(a) is a diagram to which reference is made in explaining theprinciple of the measurement of the deviation between the rotationcenter of the disk and the pattern center in the embodiment 1 of themagnetic recorder according to the invention; and FIG. 2(b) is a circuitblock diagram for the measurement of the deviation.

FIG. 3 shows data format of servo information in the embodiment 1 of themagnetic recording medium according to the invention.

FIG. 4 is a circuit block diagram for generating virtual tracks andproducing a positional error signal for tracking in the embodiment 1 ofthe magnetic recorder according to the invention.

FIG. 5 is a flowchart for the operations from the generation of virtualtracks to the tracking of the head on the virtual tracks in theembodiment 1 of the magnetic recording medium according to theinvention.

FIG. 6(a) is a plan view of main part showing the disk surface regionsfor the measurement of the deviation between the rotation center of thedisk and the pattern center in the embodiment 2 of the magnetic recorderaccording to the invention; and FIG. 6(b) is a graph schematicallyshowing the reproduced waveform from a reference track.

FIG. 7 shows data format of servo information in the embodiment 3 of themagnetic recording medium according to the invention.

FIG. 8 is a circuit block diagram for generating virtual tracks andproducing a positional error signal for tracking in the embodiment 3 ofthe magnetic recorder according to the invention.

FIG. 9 is a flowchart for repairing the virtual tracks when the rotationcenter of the disk is shifted in the embodiment 4 of the magneticrecorder according to the invention.

FIG. 10 is a flowchart for repairing the virtual tracks when therotation center of the disk is shifted in the embodiment 5 of themagnetic recorder according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described with reference to theaccompanying drawings.

[Embodiment 1]

FIGS. 1, 2, 3, 4 and 5 show diagrams for the embodiment 1 of themagnetic recorder according to the invention. As illustrated in FIG.1(a), the rotation center 11 of spindle on the magnetic disk 10 isassumed to be separated by the deviation e from the center 12 of circlesof the servo pattern recorded on the magnetic disk.

The center line 13 of the m-th data track and center line 14 of the(m+1)-th data track of the servo pattern are thus eccentric while thedisk is rotating. For example, the track center 13 is detected by theservo pattern within each of the n-th servo sector 15a, (n+1)-th servosector 15b, (n+2)-th servo sector 15c and (n+3)-th servo sector 15d (thefollowings are omitted) of m-th data track, so that a locus to be tracedby the head 16 can be found. Here, if the position of the head 16 isfixed on the absolute coordinates by maintaining the current in a voicecoil motor for driving the head 16 to be constant, the amplitude of thehead movement is 0 on the absolute coordinates as shown in FIG. 1(b). InFIG. 1(a), a circle concentric with the rotation center of the disk is alocus 17 of the head. In this invention, this locus is treated as m-thvirtual data track.

In this case, as shown in FIG. 1(c), the head traces a sinusoidal locusrelative to the center of the m-th track. More specifically, deviationsy1, y2, y3 and y4 are produced at the n-th servo sector, (n+1)-th servosector, (n+2)-th servo sector and (n+3)-th servo sector, respectively.According to the invention, the deviation at each servo sector ispreviously measured and stored. When data is recorded or reproduced, thelocus 17 of the head, or the m-th virtual data track in this inventioncan be derived from the information of each servo sector and thedeviation data.

Referring to FIG. 2(a), if the rotation center 11 of the disk isdeviated by e from the pattern center 12, the period T(n) between then-th sector and the (n+1)-th sector can be expressed by ##EQU1## fromthe figure, where λ is the wavelength between sectors on the disk, ω isthe angular velocity of rotation of the disk, and r(n) is the radiusfrom the rotation center. The radius r(n) can be found from thisequation and by measuring the period between sectors.

FIG. 2(b) is a circuit block diagram for the above processing. Theperiod between sector marks is measured by a counter 21, the aboveequation is computed by a CPU 22, and then the results from the computerare fed through a reference table 23 to a memory 24 as information foreach sector. Here, on the reference table 23, there are stored theradius r(n) and the track number of each track so that they areassociated with each other when the pattern center and the rotationcenter are assumed to be coincident.

FIG. 3 shows the data format of servo information to be stored in thememory 24 illustrated in FIG. 2(b). The servo information 30 is formedof a sector number 31 of 9 bits, a track number 32 of 16 bits andpositional error information 33 of 4 bits. Here, the positional errorinformation 33 within the range of the track width is obtained throughthe processes in which the positional error signal used in the prior artis 2 4 divided and converted into a digital form. This result isproduced from the A/D converter 25 shown in FIG. 2(b).

FIG. 4 is a circuit block diagram for generating virtual tracks andproducing a positional error signal for tracking in the embodiment 1 ofthe magnetic recorder according to the invention. Fundamentally, thedeviation data stored in the memory 42, after the sector mark and analogpositional information acquired from each servo pattern are processed bythe eccentricity measurement 20 in FIG. 2(b), is compared with thepositional error information produced by the head that traces thevirtual track, and feedback control is performed so that the differencecan be reduced to 0. The positional error information on the virtualtrack is the sum of the analog deviation in the radius direction intowhich the difference between the track number of the virtual track andthat of a certain track at which the head is desired to be located isconverted from the digital value, and the analog positional informationacquired from the servo pattern.

FIG. 5 is a flowchart for the operations from the generation of virtualtracks and the virtual track tracing of the head according to theinvention. First, the deviation is measured after the disk is loaded,and the deviation data is stored in the memory. Actually, wheninformation is magnetically recorded or reproduced, the head iscontrolled in a feedback manner so as to reduce the difference betweenthe positional error information acquired by the head at each sector andthe deviation data stored in the memory to zero, and thus located at thevirtual track.

[Embodiment 2]

FIG. 6 shows embodiment 2 of the magnetic recording medium according tothe invention. Here, the deviation measurement in the embodiment 1 ofthe magnetic recorder is replaced by the track counting referring to thereference tracks formed on the disk.

In FIG. 6(a), a magnetic disk 60 has formed thereon a reference trackregion 61 where positional information is written at the same intervalsas the track pitch Tp of data tracks, and a data region 62. When thecenter of circles of the track pattern and the rotation center of thedisk are not coincident, the reference track pattern is reproduced withthe head being fixed, and the positional error information isdemodulated, thereby producing a signal shown in, for example, FIG.6(b). The deviation can be obtained by counting points 64 at which thissignal wave intersects a certain threshold 63.

The operations after determining the deviation are the same as in theembodiment 1 of the magnetic recorder of the invention, and thus willnot be described.

[Embodiment 3]

FIGS. 7 and 8 show diagrams for the embodiment 3 of the magneticrecorder according to the invention.

The embodiment 3 of the recorder does not employ the deviation data inthe embodiment 1, but utilizes the track number and a positional errorsignal within the range of track width.

FIG. 7 shows the data format of the deviation data to be stored in thememory 24 illustrated in FIG. 2(b). The deviation data 70 is formed of asector number 71 of 9 bits, eccentric track number 72 of 16 bits, andpositional error information 73 of 4 bits. Here, the positional errorinformation 73 is produced through the processes in which the positionalerror signal used in the prior art within the range of track width is 24 divided and converted into an analog form from the digital value.

FIG. 8 is a circuit block diagram for generating virtual tracks andproducing the positional error signal for tracking. First, the analogpositional error information acquired from the servo pattern and thetrack number information for each sector are stored in the memory 24.The track number of the virtual track acquired by the head is comparedwith the track number stored in the memory 24, so that seek operation isperformed for a predetermined track. Then, feed back control is made onthe head position so as to reduce the difference between the analogpositional error signal on the virtual track acquired by the head andthat stored in the memory 24 to zero.

In the above embodiments 1 to 3, the deviation information or the servoinformation on virtual tracks may be stored in a memory such assemiconductor RAM or EEPROM. Also, it may be stored in a disk itself. Inthis case, a semiconductor memory for positioning is particularly notnecessary.

[Embodiment 4]

FIG. 9 shows the embodiment 4 of the magnetic recording medium accordingto the invention. By utilizing the methods described in the aboveembodiments, it is possible to repair the virtual tracks even though thefirst disk rotation center is deviated due to the fall of the apparatusor the like. FIG. 9 is a flowchart for this purpose.

First, when the apparatus is powered, decision is made of whether thehead traces a proper virtual track or not. Here, the proper virtualtrack is the track at which the head does not vibrate on the absolutecoordinates as shown in FIG. 1(b).

For this decision, it is possible to reproduce the reference trackpattern shown in FIG. 6(b), measure the deviation, and compare it withthe deviation produced when the proper virtual track is traced.

Another method for this decision is that when the head is located at acertain virtual track, it is examined if the current flowing in thevoice coil motor is a threshold or above.

In this method, if the virtual track is necessary to be repaired, thevirtual track is rewritten according to an virtual track rewritingroutine 91. First, the deviation data for a new virtual track isgenerated by the method described in the embodiments 1 to 3, and storedin the memory (S1). Here, the memory is a semiconductor memory or acontinuously free region of a magnetic disk. Then, the data informationon the previous virtual track is transferred to the memory (S2).Finally, the data information is written back in the new virtual trackon the basis of the deviation data on the new virtual track (S3). Whenthe amount of data is larger than the memory capacity, the entiresurface of the disk is divided into several zones, and the virtual trackrewriting routine 91 is repeatedly executed because all data cannot berewritten at a time.

[Embodiment 5]

FIG. 10 is a flowchart for the embodiment 5 of the magnetic recordingmedium according to the invention. In the embodiment 4, the rotationcenter of the disk is sometimes deviated too much for the disk to followthe tracks. In this case, the virtual tracks are repaired as follows.

First, as in the embodiment 4, decision is made of whether the headtraces a proper virtual track when the apparatus is powered. Here, theproper virtual track is the track at which the head does not vibrate onthe absolute coordinates as shown in FIG. 1(b).

If it is decided that the virtual track is necessary to be repaired, andthat the head cannot follow the virtual track itself, the deviation iscalculated by the same method mentioned in the embodiment 1 (S4). Then,feed-forward control is made on the basis of the servo informationacquired from the sum of the calculated deviation data and the servoinformation of the previous virtual track (S5). At this stage, since thehead can follow the previous virtual track, the virtual track isrepaired according to the virtual track rewriting routine 91 describedin the embodiment 4.

[Embodiment 6]

In a magnetooptical recorder, the disk is a magnetooptical recordingmedium, the head is an optical head, and the other elements are the sameas in the magnetic recorder. Therefore, the virtual tracks are set andused in the same way as in the embodiments 1 to 3 of the magneticrecorder.

[Embodiment 7]

In a magnetooptical recorder, the disk is a magnetooptical recordingmedium, the head is an optical head, and the other elements are the sameas in the magnetic recorder. Therefore, when the rotation center of thedisk is deviated from the initial position, the virtual tracks arerepaired in the same way as in the embodiments 4 and 5 of the magneticrecorder.

INDUSTRIAL APPLICABILITY

In the magnetic recorder or magnetooptical recorder of the invention,since the head need not follow faithfully the track center specified bythe servo information that is previously recorded on the magneticrecording medium or magnetooptical recording medium, a vibrationsynchronized with rotation is not caused easily.

Therefore, when the rotation center is deviated from the pattern center,since a vibration synchronized with the rotation of the disk does notoccur, the magnetic head of the magnetic recorder or the magnetoopticalhead of the magnetooptical recorder can follow the tracks with highprecision. Thus, the invention is suited to the high-density trackformation in the embedded-servo type apparatus.

What is claimed is:
 1. A magnetic recorder comprising:a magneticrecording medium having servo information and data informationmagnetically recorded; a magnetic head for recording or reproducing saidservo information and data information; means for driving said magneticrecording medium to rotate around a certain point on said medium; meansfor moving said magnetic head on said magnetic recording medium on thebasis of said servo information; means for determining an eccentricitybetween the rotation center of said magnetic recording medium and thecenter of concentric circular servo tracks on the basis of said servoinformation; means for generating deviation data from said determinedeccentricity; means for storing said deviation data; and means forsetting virtual tracks so that the rotation center of said magneticrecording medium or the vicinity thereof coincides with the track centerof said data information, and making positioning servo control on thebasis of said virtual tracks.
 2. A magnetic recorder according to claim1, wherein said eccentricity determining means computes saideccentricity on the basis of said servo information that are equally anddiscretely arranged in the rotation direction of said magnetic recordingmedium.
 3. A magnetic recorder according to claim 1, wherein saideccentricity determining means computes said eccentricity on the basisof a special pattern arranged on part of said magnetic recording medium.4. A magnetic recorder according to claim 1, wherein said deviation datais formed of sector information indicating the positions of sectors,positional error information indicating the center of track, anddeviation information at said positions of sectors.
 5. A magneticrecorder according to claim 1, wherein said deviation data storing meansis a volatile semiconductor memory provided within said magneticrecorder.
 6. A magnetic recorder according to claim 1, wherein saiddeviation data storing means is a nonvolatile semiconductor memoryprovided within said magnetic recorder.
 7. A magnetic recorder accordingto claim 1, wherein said deviation data storing means is said magneticrecording medium on which said deviation data is magnetically recorded.8. A magnetic recorder according to claim 1, wherein the center line ofeach of said virtual tracks is made coincident with the locus of saidmagnetic head by keeping a voice coil motor fixed, said voice coil motorbeing used as means for moving said magnetic head on the surface of saidmagnetic recording medium.
 9. A magnetic recorder comprising:a magneticrecording medium having servo information and data informationmagnetically recorded; a magnetic head for recording or reproducing saidservo information data information; means for driving said magneticrecording medium to rotate around a certain point on said medium; meansfor moving said magnetic head on said magnetic recording medium on thebasis of said servo information; means for generating on the basis ofsaid servo information track deviation data that is formed of sectorinformation indicating the positions of sectors, positional errorinformation indicating the center of track, and eccentric track numberinformation indicating the positions of tracks; means for storing saiddeviation data; and means for setting virtual tracks so that therotation center of said magnetic recording medium or the vicinitythereof coincides with the center of tracks of said data information onthe basis of said stored information, and making positioning servocontrol on the basis of said virtual tracks.
 10. A magnetic recorderaccording to claim 9, wherein said deviation data storing means is avolatile semiconductor memory provided within said magnetic recorder.11. A magnetic recorder according to claim 9, wherein said deviationdata storing means is a nonvolatile semiconductor memory provided withinsaid magnetic recorder.
 12. A magnetic recorder according to claim 9,wherein said deviation data storing means is said magnetic recordingmedium on which said deviation data is magnetically recorded.
 13. Amagnetic recorder according to claim 9, wherein the center line of eachof said virtual tracks is made coincident with the locus of saidmagnetic head by keeping a voice coil motor fixed, said voice coil motorbeing used as means for moving said magnetic head on the surface of saidmagnetic recording medium.
 14. A magnetic recorder comprising:a magneticrecording medium having servo information and data informationmagnetically recorded; a magnetic head for recording or reproducing saidservo information and data information; means for driving said magneticrecording medium to rotate around a certain point on said medium; meansfor moving said magnetic head on said magnetic recording medium on thebasis of said servo information; means for determining an eccentricitybetween the rotation center of said magnetic recording medium and thecenter of concentric circular servo tracks on the basis of said servoinformation; means for generating deviation data from said determinedeccentricity; means for storing said deviation data; and means forsetting virtual tracks so that the rotation center of said magneticrecording medium or the vicinity thereof coincides with the track centerof said data information, and making positioning servo control on thebasis of said virtual tracks, and when the rotation center of saidmagnetic recording medium is shifted after said virtual tracks are set,new virtual tracks are set and said positioning servo control isperformed.
 15. A magnetic recorder according to claim 14, wherein whensaid rotation center is shifted, said data information is transferred toa continuous free region on said magnetic recording medium, and then newvirtual tracks are set.
 16. A magnetic recorder according to claim 14,wherein when said rotation center is shifted, said data information istransferred to a semiconductor memory provided within said magneticrecorder, and then new virtual tracks are set, followed by the writingback of said data information.
 17. A magnetic recorder, comprising:amagnetic recording medium having servo information and data informationmagnetically recorded; a magnetic head for recording or reproducing saidservo information and data information; means for driving said magneticrecording medium to rotate around a certain point on said medium; meansfor moving said magnetic head on said magnetic recording medium on thebasis of said servo information; means for determining an eccentricitybetween the rotation center of said magnetic recording medium and thecenter of concentric circular servo tracks on the basis of said servoinformation; means for generating deviation data from said determinedeccentricity; means for storing said deviation data; means for settingvirtual tracks so that the rotation center of said magnetic recordingmedium or the vicinity thereof coincides with the track center of saiddata information, and making positioning servo control on the basis ofsaid virtual tracks, and when the rotation center of said magneticrecording medium is shifted after said virtual tracks are set, forsetting new virtual tracks; and means for transferring track datainformation stored on said virtual tracks before said rotation center isshifted to a continuous free region on said magnetic recording medium orto a semiconductor memory provided within said magnetic recorder, andafter said new virtual tracks are set, transferring said track datainformation to said new virtual tracks from said continuous free regionor said semiconductor memory.